Fix rails command

Migrations should be generated in the extension dir, not in the dummy
app
Group inventory units by line item on shipment manifest

Update Package::ContentItem to also have a line item.
Resolve conflich between individual product and bundle part

Spree should know whether the inventory unit is associated with a
product sold individually or with a part of a bundle.

We probably need to also add a part column on the InventoryUnit table so
that this behaviour is persisted at the unit level doesn't depend on the
bundle product state which might change in the future
Scope uniq line item assemblies only